Microlithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components. This involves using a mask (reticle) that carries the pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component. A mask is positioned into a projection exposure apparatus between illumination system and projection objective in the region of the object surface of the projection objective and illuminated with an illumination radiation provided by the illumination system. The radiation altered by the mask and the pattern passes as projection radiation through the projection objective, which images the pattern of the mask onto the substrate to be exposed, which normally carries a radiation-sensitive layer (photoresist).
In the case of projection microlithography, the mask is illuminated with the aid of an illumination system, which shapes from the light from a primary light source, such as a laser, illumination radiation that is directed onto the mask and is defined by specific illumination parameters. The illumination radiation impinges on the mask within an illumination field (area of defined shape and size, for example, rectangular field or curved ring field), wherein the shape and size of the illumination field are generally constant (not variable). An intensity distribution that is as uniform as possible is generally sought within the illumination field, for which purpose homogenizing devices, for example, light mixing elements such as fly's eye condensers and/or rod integrators, can be provided within the illumination system.
Moreover, depending on the type of structures to be imaged, different illumination modes (so-called “illumination settings”) are often involved, which can be characterized by different local intensity distributions of the illumination radiation in a pupil surface of the illumination system. In this context, this is sometimes called “structured illumination” or “structuring of the illumination pupil” or structuring of the secondary light source. The pupil surface of the illumination system in which specific, definable two-dimensional intensity distributions (the secondary light sources) are intended to be present is also referred to in this application as “pupil shaping surface”, since essential properties of the illumination radiation are generally “shaped” with the aid of this intensity distribution. The illumination settings include, for example, in the case of the conventional illumination settings, round illumination spots centered around the optical axis of the illumination system and having different diameters (generally defined by the degree of coherence σ of the illumination) and, in the case of non-conventional, i.e. abaxial types of illumination, ring illumination (or annular illumination) and also polar intensity distributions, for example dipole illumination or quadrupole illumination. The non-conventional illumination settings for generating an abaxial (oblique) illumination may serve, among other things, to increase the depth of focus by two-beam interference and to increase the resolution.
In the case of an illumination system incorporated into a microlithography projection exposure apparatus, the “pupil shaping surface” of the illumination system, in which the desired two-dimensional intensity distribution (secondary light source) is intended to be present, can be situated at or near a position which is optically conjugate with respect to a pupil plane of a downstream projection objective. In general, the pupil shaping surface can correspond to a pupil surface of the illumination system or lie in the vicinity thereof. Provided that the intervening optical components do not change the ray angle distribution, that is to say operate in angle-maintaining fashion, the angle distribution of the illumination radiation impinging on the pattern of the mask is determined by the spatial intensity distribution in the pupil shaping surface of the illumination system. Moreover, provided that the intervening optical components operate in angle-maintaining fashion, the spatial intensity distribution in the pupil of the projection objective is determined by the spatial intensity distribution (spatial distribution) in the pupil shaping surface of the illumination system.
Those optical components and assemblies of the illumination system which are provided for receiving light from a primary light source, for example a laser or a mercury vapor lamp, and for generating therefrom a desired two-dimensional intensity distribution (secondary light source) in the “pupil shaping surface” of the illumination system jointly form a pupil shaping unit, which should generally be variably adjustable.
US 2007/0165202 A1 (corresponding to WO 2005/026843 A2) in the name of the applicant discloses illumination systems in which a pupil shaping unit for receiving light from a primary light source and for generating a variably adjustable two-dimensional intensity distribution in a pupil shaping surface of the illumination system includes a multimirror array (MMA) with individually drivable individual mirrors that can alter the angle distribution of the radiation incident on the mirror elements in a targeted manner such that the desired illumination intensity distribution results in the pupil shaping surface.
Methods for calculating optimum structurings of the intensity distribution in the pupil shaping surface of an illumination system in a manner dependent on mask structures to be imaged are disclosed for example in U.S. Pat. No. 6,563,556 or US 2004/0265707.